The present invention relates to a ceramic-made electrostatic chuck or, more particularly, to a ceramic-made electrostatic chuck capable of electrostatically attracting and holding semiconducting and insulating work pieces irrespective of the temperature in a wide range and still capable of quickly releasing the work piece held thereon when the applied voltage is turned off so as to be useful in the manufacturing process of various kinds of semiconductor devices, liquid crystal display units and the like.
It is a trend in the manufacturing process in the electronic industry of recent years that the traditional wet-process methods are under continuous replacement with dry-process methods such as dry etching, ion implantation, sputtering and the like in view of the higher adaptability of the dry-process to automatization. Since these dry-process procedures are conducted mostly in a vacuum chamber, it is very important to ensure high exactness of positioning of the work piece such as a semiconductor silicon wafer, glass plate and the like on a correct position within a vacuum chamber. This problem is more important in recent years because the size or diameter of the work pieces such as silicon wafers, glass plates is increasing year by year along with an increase in the density of circuit integration and fineness of patterning on the work pieces.
A traditional means for the transportation and holding of a work piece is a vacuum chuck having a perforated chucking plate communicated with a vacuum line so as to attract and hold a work piece by means of the pressure difference between the atmospheric pressure and the reduced pressure inside the chucking head. As is readily understood from the above described working principle, vacuum chucks cannot be used in a vacuum chamber for dry-processing. Although vacuum chucks can be used under non-vacuum conditions, vacuum chucks are accompanied even there by a serious problem that, since the attracting force in a vacuum chuck is localized necessarily on and around the perforations, any work piece attracted to a vacuum chuck is unavoidably in an unevenly stressed state so that vacuum chucks cannot be used when extremely high precision is required in the positioning of work pieces. Accordingly, vacuum chucks have no wide applicability in the modern electronic industry.
In place of the above described vacuum chucks, accordingly, electrostatic chucks, in which work pieces are attracted to and held by a chucking head by means of an electrostatic force, are highlighted in the electronic industry as a transportation and holding means of work pieces. Besides the use as a transportation and holding means, studies are now under way for the use of the attracting force of an electrostatic chuck as a means for the correction of flatness of semiconductor wafers and glass plates as a substrate of liquid crystal display units in view of the increasing importance of the flatness thereof along with the increase in the fineness of working thereon. Because a very large electrostatic attracting force is required for the flatness correction of substrate plates, various proposals and attempts have been made therefor. For example, a proposal has been made in Japanese Patent Kokai 62-94953, 2-206147, 3-147843 and 3-204924, according to which the ceramic base plate, on which the electrodes are formed from a conductive material such as a metal, is prepared with admixture of titanium dioxide so as to decrease the volume resistivity of the ceramic base plate with a consequent increase in the electrostatic attracting force.
When an alumina-based ceramic base plate is prepared with admixture of titanium dioxide, the volume resistivity of the ceramic material is decreased to cause a small electric current therethrough with an increase in the electrostatic attracting force by the Johnsen-Rahbeck effect but a serious problem is caused thereby that, since titanium dioxide is a semiconductive material, a great decrease is caused in the response characteristics to the application and removal of the electric voltage even with an appropriate volume resistivity so that the times taken for reaching the maximum attracting force after application of an electric voltage and taken for the disappearance of the attracting force after removal of the electric voltage are greatly extended. This problem is more serious when the working temperature of the electrostatic chuck is low. In addition, the volume resistivity of an alumina-based ceramic material can be decreased only by the admixture of a quite large amount of titanium dioxide. For example, the volume resistivity of an alumina-based ceramic material can be decreased to 1.times.10.sup.8 ohm.cm by the admixture of about 25% by weight of titanium dioxide. In the manufacturing process of semiconductor devices, however, it is generally unpreferable that any body having a possibility of coming into contact with the semiconductor material contains titanium because titanium is sometimes a detrimental dopant against the performance of semiconductor devices. When the semiconductor wafer as the work piece to be attracted by the electrostatic chuck is at room temperature or higher, moreover, the volume resistivity is so low that an unduly large leak current is caused to eventually damage the circuit on the substrate wafer.